The present invention relates to a single plate type color liquid crystal display apparatus featuring high efficiency achieved by utilizing light diffraction, and other products in which this invention is applied, and more particularly to a projection type display apparatus.
To help the present invention better understood, FIG. 1 summarizes the differences of the present invention from the prior art. In FIG. 1, document (A) is the present inventor's prior application titled "High Efficiency Liquid Crystal Display" U.S. Pat. No. 5,355,189 and U.S. Pat. No. 5,537,171, document (B) concerns "a display apparatus" (JP-A-6-230384), document (C) is a thesis titled "Compact Spatio-Chromatic Single-LCD Projection Architecture" by B. A. Loiseaux. et al, Asia Display. '95, P87-P89, and document (D) is another thesis titled "Holographic Optical Element for Liquid Crystal Projector" by N. Ichikawa, Asia Display. '95, P727-P729.
These techniques of the prior art, as indicated in Item 2.1 in FIG. 1, use diffracting plate means which have a flat plate as viewed at a macroscopic level and also includes diffracting plate means minutely formed at a microscopic level. FIG. 2 shows the function of the above-mentioned macroscopically flat diffracting plate means.
In FIG. 2, reference numeral 1 denotes single plate type color liquid crystal panel means, 2 denotes three-primary-color pixels, 3 denotes light-input-side lenses forming three-position means for converging input three rays of each primary color coming from three directions to respective color positions at every triopixel pitch, 23 denotes a block collectively representing a light source and collimator means. For the functions of the above-mentioned components, refer to the descriptions in technique (A) in prior art in FIG. 1. Reference numeral 5 denotes macroscopically flat diffracting plate means, 6 denotes a diffraction grating formed on the outgoing plane of the diffracting plate means 5, 7 denotes incident white light, and 7' denotes the direction of a zero-order light output from the diffracting plate. Normally, such a structure of the diffraction grating is selected as to minimize the power of the zero-order light. R, G and B represent red, green and blue light rays, but they may sometimes be referred to as R ray and so on for short. Reference numerals 8, 8' and 8" denote G ray, R ray and B ray of the first-order diffracted light output from the diffracting plate. The angle .omega. denotes the angle of deflection of G ray by the diffracting plate, and denotes a difference between the angle of deflection of R ray and that of G ray caused by the diffracting plate and is hereafter referred to as "R-G separation angle."
Next, with regard to Item 3.2 in FIG. 1, the reason why the deflection angle of G ray has to be 20 or larger in the prior art will be described with reference to FIG. 3.
In FIG. 3, reference numerals 6, 7 and 8 and 8 and .omega. denote the same things as have been described. The symbol .alpha.1 denotes the angle of incidence of the incident light to the diffraction grating 6, .epsilon..sub.1 denotes the angle of divergence of the incident light, .alpha..sub.2 and .epsilon..sub.2 denote the incident and diverging angles of the first-order diffracted green outgoing light, .alpha.'.sub.2 denotes the incident angle of the red outgoing light, and P.sub.0 denotes the pitch (period of array) of the diffraction grating.
In compliance with the principle of light diffraction, the following equation holds. ##EQU1## where .lambda..sub.G .apprxeq.530 nm (G ray wavelength) .lambda..sub.R .apprxeq.610 nm (R ray wavelength) ##EQU2##
That is to say, it is known that the magnitude of the G-R separation angle .omega. is subordinately constrained by the green deflection angle .delta..sub.G, and limited to about 15% of .delta..sub.G.
On the other hand, attention needs to be paid to a fact that the relation between the diverging angles .epsilon..sub.1 and .epsilon..sub.2 can be obtained by differentiating equation (1). ##EQU3##
To separate G ray and R ray, the following condition must be satisfied. ##EQU4##
Generally, according to the law of energy conservation, if the effective surface area of a Lambertian light source is designated by A.sub.0 and the vertical projection of the incident area of the diffracting plate is designated by A.sub.1, the diverging angle .epsilon..sub.1 of the incident light is given by the following expression. ##EQU5##
If the diverging angle distribution is uniform, equation (8) holds. In actuality, however, the diverging angle distribution is not uniform, so that inequality (9) holds. Normally, the maximum value .epsilon..sub.1MAX of .epsilon..sub.1 is about 1.5 times as large as .epsilon..sub.1 in most cases. The minimum projection light source proposed as a test case in the past is about 1.4 mm.phi. and therefore the surface area is .pi. (1.4 mm).sup.2, namely, about 6.3 (mm).sup.2. The surface area of the ordinary projection light source is about 25 (mm).sup.2. On the other hand, the area of the diffracting plate is substantially equal to the area of the liquid crystal panel, and the diffracting plate of the maximum size for projection use has an opposite angle of about 6.5" and an area of 13,000 (mm)2. By substituting those values into expression (9), we have the following expression. ##EQU6##
As is obvious from the above expression, it is very difficult in the prior art to set the deflection angle .delta..sub.G to be 10.degree. less when the R-G separation angle is 0.05 rad or larger.
From the above explanation, the description of the prior art in Item 3.2 in FIG. 1 can be understood.
The ground for the description of the prior art in Item 3.4 in FIG. 1 will be demonstrated in the following.
Before that, as the first step, it is necessary to describe the principle of prior art techniques (B) and (D) related to Item 3.3 in FIG. 1. Description will be made with reference to FIG. 4.
In FIG. 4, reference numerals 1 and 2 denote the same things as have been described, 11, 12 and 13 denote incident light rays, 11', 12' and 13' denote the outgoing directions of zero-order light rays, and 6' denotes a modulated pitch macroscopically flat diffraction grating. The symbol .omega. denotes the above-mentioned R-G separation angle, T denotes a pitch of trio pixel array, and .delta..sub.1, .delta..sub.2 and .delta..sub.3 denote lower, middle and upper deflection angles of respective first-order diffraction G rays. As illustrated, the deflection angles are modulated differently at each pitch of trio pixel array. Therefore, in the conventional techniques (B) and (D) in FIG. 1, it is implicitly proposed to modulate the pitch of the diffraction grating according to the above-mentioned equation (1). Therefore, from the relation in FIG. 1 and equation (1), the relation of the following expression needs to be satisfied.
In the following expressions, P.sub.01, P.sub.02 and P.sub.03 are the pitches of those portions of the diffraction grating which correspond to the .delta..sub.1, .delta..sub.2 and .delta..sub.3. ##EQU7##
By the above analysis, the following could be clarified. Specifically, if the pitch of the diffraction grating is modulated in synchronism with the period of trio pixel array to satisfy equations (13) and (14), the diffraction grating 6' can be made to also play the role of the three-position means 3 in FIG. 2 so long as G ray is concerned. With this, description of the result of clarification by the present inventor of the principles of the conventional techniques related to Item 3.3 in FIG. 1 is finished.
As the second step, description will proceed to the conventional technique in Item 3.3 in FIG. 1. In the macroscopically flat plate diffraction grating, there is a condition of constraint shown in equation (4), which has been described above. From equations (4), (1) and (15), it is inevitable that an unfavorable expression given below holds. ##EQU8##
In the above equation, .omega..sub.01 and .omega..sub.03 are R-G separation angles by those portions of the diffraction grating which correspond to .delta..sub.1 and .delta..sub.3 in FIG. 4. The relation represented by the above expression is shown in FIG. 5.
If in the above expression the value is 1.0 instead of 1.6, all three rays, including the red ray and the blue ray, besides the green ray, are matched (coincide) with the liquid crystal elements. However, because the above value is 1.6, the red ray and the blue ray are not matched with the target liquid crystal elements as shown in FIG. 5. In other words, the utilization efficiency of the red and blue light ray deteriorates.
FIG. 5 shows only one period of the trio pixel array. In FIG. 5, reference numerals 2 and 6' denote same things as already described, 14, 15 and 16 denote the first order diffracted G rays, 17, 18 and 19 denote the first order diffracted R rays, and 20, 21 and 22 denote the first order B rays. As is clear from FIG. 5, R and B component rays of the principal ray striking the center of each pitch of the diffraction grating 6' are both correctly incident on the target pixels. On the other hand, the marginal rays corresponding to both end portions of each pitch are unable to be incident on the target pixels due to deviation loss. Therefore, in the techniques (B) and (D) in the prior art, the problem has become evident that the convergence of R and B rays to the red and blue pixels by the diffraction grating is not compatible as mentioned in Item 3.4 in FIG. 1. More specifically, the problem includes the deterioration of the transmission efficiency of R and B rays and the deterioration of chromatic purity owing to color mixture.
In the course of the above description, the constraint on the size of a usable light source was mentioned, and it is vitally important to take this constrain into consideration.
This is because not only a point light source such as can be expressed as a mathematical concept of a point does not exist in reality, but it should be understood that a point light source is incapable of existing according to the principle of uncertainty or the second law of thermodynamics. A premise including only one wrong hypothesis would lead to illusions such that innumerable impossibilities are taken for possibilities. For example, if a viability of a point light source is supposed, from this it may be deduced that thermonuclear power generation can be realized easily.
Therefore, the above-mentioned constraint on the size of the light source is an important matter requiring consideration.
The present invention could not have been possible without the above-mentioned result of clarification by the present inventor. On account of this, this subject will first be summarized in the following.